题目连接
http://acm.hdu.edu.cn/showproblem.php?pid=5154
Harry and Magical Computer
Description
In reward of being yearly outstanding magic student, Harry gets a magical computer. When the computer begins to deal with a process, it will work until the ending of the processes. One day the computer got n processes to deal with. We number the processes from 1 to n. However there are some dependencies between some processes. When there exists a dependencies (a, b), it means process b must be finished before process a. By knowing all the m dependencies, Harry wants to know if the computer can finish all the n processes.
Input
There are several test cases, you should process to the end of file.
For each test case, there are two numbers n m on the first line, indicates the number processes and the number of dependencies. $1 leq n leq 100,1 leq m leq 10000$
The next following m lines, each line contains two numbers a b, indicates a dependencies $(a, b). 1 leq a, b leq n$
Output
Output one line for each test case.
If the computer can finish all the process print "YES" (Without quotes).
Else print "NO" (Without quotes).
Sample Input
3 2
3 1
2 1
3 3
3 2
2 1
1 3
Sample Output
YES
NO
先建立一张有向图，再遍历，若有顶点未访问到输出"NO",否则输出"YES"。。。
1 #include<algorithm> 2 #include<iostream> 3 #include<cstdlib> 4 #include<cstring> 5 #include<cstdio> 6 #include<vector> 7 #include<queue> 8 #include<map> 9 using std::cin; 10 using std::cout; 11 using std::endl; 12 using std::find; 13 using std::sort; 14 using std::pair; 15 using std::vector; 16 using std::queue; 17 using std::multimap; 18 #define pb(e) push_back(e) 19 #define sz(c) (int)(c).size() 20 #define mp(a, b) make_pair(a, b) 21 #define all(c) (c).begin(), (c).end() 22 #define iter(c) decltype((c).begin()) 23 #define cls(arr,val) memset(arr,val,sizeof(arr)) 24 #define cpresent(c, e) (find(all(c), (e)) != (c).end()) 25 #define rep(i, n) for (int i = 0; i < (int)(n); i++) 26 #define tr(c, i) for (iter(c) i = (c).begin(); i != (c).end(); ++i) 27 const int N = 110; 28 int n, m, G[N][N]; 29 bool vis[N], flag[N]; 30 typedef unsigned long long ull; 31 void bfs() { 32 bool f = false; 33 queue<int> que; 34 rep(i, n) { 35 if (!flag[i]) { 36 vis[i] = true; 37 que.push(i); 38 } 39 } 40 while (!que.empty()) { 41 int p = que.front(); que.pop(); 42 rep(i, n) { 43 if (G[p][i]) { 44 if (vis[i]) continue; 45 que.push(i); 46 vis[i] = true; 47 } 48 } 49 } 50 rep(i, n) { 51 if (!vis[i]) { f = true; break; } 52 } 53 puts(f ? "NO" : "YES"); 54 } 55 int main() { 56 #ifdef LOCAL 57 freopen("in.txt", "r", stdin); 58 freopen("out.txt", "w+", stdout); 59 #endif 60 int a, b; 61 while (~scanf("%d %d", &n, &m)) { 62 cls(vis, 0), cls(flag, 0), cls(G, 0); 63 rep(i, m) { 64 scanf("%d %d", &a, &b); 65 G[b - 1][a - 1] = 1; 66 flag[a - 1] = 1; 67 } 68 bfs(); 69 } 70 return 0; 71 }